ArcGIS Pro

Geographic vs Projected Coordinate Systems

When you are trying to choose a coordinate system for your map, are you sometimes confused by the options?

Geographic and Projected Coordinate System groups in the available coordinate systems list

What is the difference between a geographic coordinate system (GCS) and a projected coordinate system (PCS) anyways?

Here’s the short answer:

Coordinate System Details window with annotation
The same location on a GCS and a PCS

Where: Geographic Coordinate Systems

You are part of a search and rescue team looking for an injured person in the Australian outback. The point location you have from her satellite phone is 134.577°E, 24.006°S. Where is she located?

Two point locations on the imagery basemap

Both location A and B in the above image are correct. A is 134.577°E, 24.006°S in one GCS (Australian Geodetic Datum 1984) and B is the same coordinate location in another (WGS 1984). Without knowing which GCS the data is in, you don’t know if the hiker is on top of the plateau or if she has fallen off the cliff.

A geographic coordinate system (GCS) is used to define locations on a model of the surface of the earth. The GCS uses a network of imaginary lines (longitude and latitude) to define locations. This network is called a graticule.

Illustration of the graticular network

So why isn’t knowing the latitude and longitude of a location good enough to know where it is? How can location A and location B in the Australia example both be correct?

Well it turns out the earth isn’t a perfect sphere. It’s a lumpy, bumpy, and uneven rounded surface. There are high mountains and deep ocean trenches. Because the planet spins, the poles are a bit closer to the center of the earth than the equator is. But in order to draw a graticule, you need a model of the earth that is at least a regular spheroid, if not a perfect sphere.

There are many different models of the earth’s surface, and therefore many different GCS! World Geodetic System 1984 (WGS 1984) is designed as a one-size-fits-all GCS, good for mapping global data. Australian Geodetic Datum 1984 is designed to fit the earth snugly around Australia, giving you good precision for this continent but poor accuracy anywhere else.

The GCS is what ties your coordinate values to real locations on the earth. The coordinates 134.577°E, 24.006°S only tell you where a location is within a geographic coordinate system. You still need to know which GCS it is in before you know where it is on Earth.

How: Projected Coordinate Systems

Once your data knows where to draw, it needs to know how. The earth’s surface—and your GCS—are round, but your map—and your computer screen—are flat. That’s a problem. You can’t draw the round earth on a flat surface without deforming it. Imagine peeling an orange and trying to lay the peel flat on a table. You can get close, but only if you start tearing the peel apart. This is where map projections come in. They tell you how to distort the earth—how to tear and stretch that orange peel—so the parts that are most important to your map get the least distorted and are displayed best on the flat surface of the map.

Illustration of attempts to flatten a globe

You’re perhaps already aware that there are many different map projections and that each displays the earth in a different way. Some are good for preserving areas on your map, others at preserving angles or distances.

Some projections in ArcGIS

A projected coordinate system (PCS) is a GCS that has been flattened using a map projection.

Your data must have a GCS before it knows where it is on earth. Projecting your data is optional, but projecting your map is not. Maps are flat, so your map must have a PCS in order to know how to draw.

Next, let’s take a look at how these coordinate systems (geographic and projected alike) are put together.

Coordinate System Construction

In ArcGIS Pro, you can view the details of any coordinate system in the Map Properties window, on the Coordinate Systems tab. Click the green Details link.

Details link in the Map Properties > Coordinate Systems window

The image below shows the Details page for the Fuller (world) coordinate system:

Coordinate System Details for Fuller (world)
Two different PCS based on the Albers Equal Area Conic projection

Remember that a PCS is just a GCS that has been projected. Let’s look at the properties of the WGS 1984 geographic coordinate system:

But wait!

Can’t we choose a GCS for our map instead of a PCS? I’ve made maps before that were in WGS 1984 and they drew just fine, didn’t they?

Let’s try it out.

In Map Properties, expand the Geographic Coordinate System list and choose any one. Click OK.

Geographic Coordinate System group in the available coordinate systems list

Your map will look like this, regardless of which GCS you chose:

Map of the world in plate caree

Remember that it is impossible to draw the round earth on a flat surface without a projection. So when you tell ArcGIS to make a flat map with a GCS, it is forced to choose a projection! So it draws using a pseudo Plate Carrée projection. This is just latitude and longitude represented as a simple grid of squares. It is called pseudo because it is measured in angular units (degrees) rather than linear units (meters). This projection is easy to understand and easy to compute, but it also distorts all areas, angles, and distances, so it is senseless to use it for analysis and measurement. You should choose a different PCS.

Summary:

Most of the time, you don’t need to choose a GCS. Your data was already stored in one and you should just stick with that. But often you do need to choose a PCS. Read the lesson Choose the Right Projection to learn how to pick the right one.

There are plenty of other resources that help to explain coordinate systems in greater depth and in different ways. If you want to learn more, try some of these:

Cloth globe image by Charles Preppernau.

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About the author

Heather is a cartographer and artist who mixes both practices to express and understand landscapes. She writes and edits lessons for the Learn ArcGIS website. View more of her work at www.heathergabrielsmith.ca

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Joe Wokiri
Joe Wokiri

Many thanks Heather Smith. The concept of pseudo Plate Carrée projection is now clear to me.

Scott Drzyzga
Scott Drzyzga

The where vs. how framework you constructed here is useful for distinguishing geographic coordinates from their projected counterparts. Thank you for sharing it so effectively. I take issue, however, with the paragraph that defines geographic coordinate systems and graticules as equivalent (i.e., as a grid of lines). A GCS definitely is NOT a grid of lines. I’m disappointed to see this misconception presented as fact in an Esri sponsored blog because it takes so long to get students to relinquish it. A GCS is a continuous and 3-dimensional {lon, lat, AND either radius, height, or depth} coordinate system that’s useful… Read more »

Andrew Link
Andrew Link

Great explanation and examples for the definition of the GCS and PCS. This is by far the best I have read! Thank you.

Luigi Falco
Luigi Falco

Great explanation… I take advantage of your PCS knowledge to ask you which kind of projection (must be Equal area) would you use for the Ocean Indian area that EAST to WEST goes from Suez to Philippines and NORTH_SOUTH from coastline to the Equator? Thank you!

Sujata Sunuwar
Sujata Sunuwar

Thank you for explaining it so well. I understood it finally

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